Cellulose-reactive sizing agents such as those based on alkenyl succinic anhydride (ASA) are widely used in papermaking at neutral or slightly alkaline stock pH's in order to give paper and paper board some degree of resistance to wetting and penetration by aqueous liquids. Paper sizes based on cellulose-reactive sizing agents are generally provided in the form of dispersions containing an aqueous phase and finely divided particles or droplets of the sizing agent dispersed therein. The dispersions are usually prepared with the aid of a dispersant system consisting of an anionic compound, e.g. sodium lignosulfonate, in combination with a high molecular weight amphoteric or cationic polymer, e.g. cationic starch, polyamine, polyamideamine or a vinyl addition polymer.
WO 96/17127 discloses aqueous dispersions which comprise a cellulose-reactive sizing agent and colloidal anionic aluminium-modified silica particles.
WO 97/31152 discloses aqueous dispersions which comprise a reactive size and an anionic microparticulate material. The dispersions may also contain not more than 2% (by weight based on the weight of the reactive size) of surfactant. The surfactant can be non-ionic or anionic.
WO 98/33979 A1 discloses an aqueous dispersion of cellulose-reactive sizing agent and a dispersant system comprising a cationic organic compound and an anionic stabilizer.
Despite the fact that considerable improvements have been achieved in the preparation, properties and performance of aqueous dispersions of alkenyl succinic anhydride, there are still some technical problems associated with the use of such dispersions. Usually, dispersions of alkenyl succinic anhydride exhibit poor stability, which evidently leads to difficulties in handling the dispersions, for example on storage and in use. One further drawback is that the aqueous dispersions cannot be stored for longer periods of time, because alkenyl succinic anhydride hydrolyses easily and thereby becomes ineffective as a sizing agent. Therefore, the alkenyl succinic anhydride is usually delivered to paper mills as a liquid, which is then dispersed prior to its use as a sizing agent and the dispersion obtained is usually used within 2 hours to avoid the problems of insufficient stability and loss of sizing efficiency. The equipment used to prepare the dispersions provides high shear forces to be able to set surfaces free and produce dispersions having adequate particle size. Such equipment is often both complicated and expensive, and due to the high shear forces usually requires a considerable amount of energy.
It is an object of this invention to provide an aqueous dispersion of cellulose-reactive sizing agent which can be easily prepared using low shear forces and low energy consumption. It is a further object of this invention to provide an aqueous dispersion of cellulose-reactive sizing agent showing improved stability and sizing efficiency. Further objects will appear hereinafter.